This invention relates to apparatus for continuously producing photovoltaic devices on a substrate by depositing successive amorphous-silicon alloy semiconductor layers in each of at least two adjacent deposition chambers. The composition of each amorphous layer is dependent upon the particular process gases introduced into each of the deposition chambers. The gases introduced into the first deposition chamber are carefully controlled and isolated from the gases introduced into the adjacent deposition chamber. More particularly, the deposition chambers are operatively connected by a relatively narrow gas gate passageway (1) through which the web of substrate material passes; and (2) which is adapted to isolate the process gases introduced into the first chamber from the process gases introduced into the adjacent deposition chamber. As is well known in the art, despite the relatively small size of the gas gate passageways, a percentage of gases introduced into one chamber still back diffuse into the adjacent chamber, thereby contaminating the layer deposited in said adjacent chamber. In an effort to further reduce back diffusion of process gases, prior art gas gates have incorporated supply conduits at the high pressure side of the gas gates for introducing inert gases adapted to flow, at high velocities, through the gas gate passageway. While the use of the inert sweep gases was beneficial in further reducing back diffusion through the gas gate passageway, the high rate of speed with which they traveled therethrough often produced turbulent flow patterns which tended to partially increase the back flow or back diffusion of process gases, thereby reducing the efficiency of photovoltaic devices produced therefrom.
Further, although each deposition chamber includes an evacuation port adjacent the plasma region thereof for withdrawing unused process gases and nondeposited plasma, not all of the process gases and plasma can be withdrawn therethrough before they contact a wall of the deposition chamber. Process gases and plasma which contact a chamber wall form a silane powder which adheres to the semiconductor layer deposited onto the substrate. The formation of powder between layers of a semiconductor device can severely harm or destroy the efficiency of that device.
The present invention operates to: (1) substantially reduce the turbulent flow of sweep gases through the gas gate passageway; and (2) reduce the formation of powder between semiconductor layers caused by unused process gases and nondeposited plasma contacting the walls of a deposition chamber.
Recently, considerable efforts have been made to develop systems for depositing amorphous semiconductor alloys, each of which can encompass relatively large areas, and which can be doped to form p-type and n-type materials for the production of p-i-n-type devices which are, in operation, substantially equivalent to their crystalline counterparts.
It is now possible to prepare amorphous silicon alloys by glow discharge techniques which possess (1) acceptable concentrations of localized states in the energy gaps thereof, and (2) high quality electronic properties. Such a technique is fully described in U.S. Pat. No. 4,226,898, entitled Amorphous Semiconductors Equivalent to Crystalline Semiconductors, Stanford R. Ovshinsky And Arun Madan which issued Oct. 7, 1980; and by vapor deposition as fully described in U.S. Pat. No. 4,217,374, Stanford R. Ovshinsky and Masatsugu Izu, which issued on Aug. 12, 1980, under the same title. As disclosed in these patents, fluorine introduced into the amorphous silicon semiconductor layers operates to substantially reduce the density of the localized states therein and facilitates the addition of other alloying materials, such as germanium.
The concept of utilizing multiple cells, to enhance photovoltaic device efficiency, was discussed at least as early as 1955 by E. D. Jackson, U.S. Pat. No. 2,949,498 issued Aug. 16, 1960. The multiple cell structures therein discussed utilized p-n junction crystalline semiconductor devices. Essentially the concept is directed to utilizing different band gap devices to more efficiently collect various portions of the solar spectrum and to increase open circuit voltage (Voc.). The tandem cell device has two or more cells with the light directed serially through each cell, with a large band gap material followed by smaller band gap materials to absorb the light passed through the first cell. By substantially matching the generated currents from each cell, the overall open circuit voltages from each cell may be added, thereby making the greatest use of light energy passing through the semiconductor device.
It is of obvious commercial importance to be able to mass produce photovoltaic devices. Unlike crystalline silicon which is limited to batch processing for the manufacture of solar cells, amorphous silicon alloys can be deposited in multiple layers over large area substrates to form solar cells in a high volume, continuous processing system. Continuous processing systems of this kind are disclosed, for example, in pending patent applications: Ser. No. 151,301, filed May 19, 1980 for A Method of Making P-Doped Silicon Films and Devices Made Therefrom; Ser. No. 244,386, filed Mar. 16, 1981 for Continuous Systems For Depositing Amorphous Semiconductor Material; Ser. No. 240,493, filed Mar. 16, 1981 for Continuous Amorphous Solar Cell Production System; Ser. No. 306,146, filed Sept. 28, 1981 for Multiple Chamber Deposition and Isolation System and Method; and Ser. No. 359,825, filed Mar. 19, 1982 for Method And Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells. As disclosed in these applications, a substrate may be continuously advanced through a succession of deposition chambers, wherein each chamber is dedicated to the deposition of a specific material. In making a solar cell of p-i-n-type configuration, the first chamber is dedicated for depositing a p-type amorphous silicon alloy, the second chamber is dedicated for depositing an intrinsic amorphous silicon alloy, and the third chamber is dedicated for depositing an n-type amorphous silicon alloy. Since each deposited alloy, and especially the intrinsic alloy must be of high purity, the deposition environment in the intrinsic deposition chamber is isolated from the doping constituents within the other chambers to prevent the back diffusion of doping constituents into the intrinsic chamber. In the previously mentioned patent applications, wherein the systems are primarily concerned with the production of photovoltaic cells, isolation between the chambers is accomplished by gas gates through which unidirectional gas flow is established and through which an inert gas may be "swept" about the web of substrate material.
While the combination of (1) establishing a substantially unidirectional flow of gases from the intrinsic deposition chamber to adjacent dopant chambers through a small gas gate passageway; (2) reducing the size of those passageways by employing magnetic assemblies which urge the unlayered substrate surface toward one of the passageway walls; and (3) directing inert sweep gases from the high pressure side to the low pressure side of the gas gate serve to substantially reduce back diffusion of dopant process gases through the gas gate passageway and hence reduce contamination of the intrinsic semiconductor layers, it has been discovered that the velocity of the inert sweep gases flowing through the passageway must be carefully controlled to maintain the flow in a laminar state. Should the flow become turbulent, it becomes impossible to calculate the degree of back diffusion of process gases and the rate of back diffusion may actually increase. It is therefore one object of the present invention to provide apparatus which will prevent the flow of sweep gases through the gas gate passageway from becoming turbulent.
The plasma region of a deposition chamber is defined as the region between the cathode and the substrate wherein process gases are disassociated into the plasma which is then adapted to be deposited onto the substrate. The process gases are introduced into the deposition chamber adjacent the plasma region, are pulled across the top surface of the cathode and are withdrawn, along with the nondeposited plasma, through a port located at the underside of the cathode. By introducing and withdrawing the process gases and plasma adjacent the plasma region, an attempt was made by prior art apparatus to prevent the process gases and plasma from contacting the walls of the deposition chamber. However, it has been determined that not all of the process gases and plasma are immediately withdrawn. The result is that the process gases and plasma which are not immediately withdrawn are free to escape from the plasma region and contact the walls of the deposition chamber. The process gases and plasma contacting the deposition chamber walls form a silane powder which can settle between semiconductor layers deposited on the substrate. The powder either seriously impairs or shorts out a photovoltaic device produced from the semiconductor layers (particularly when the powder forms in the intrinsic deposition chamber between the p and n semiconductor layers). It is therefore another object of the present invention to flow inert sweep gases into the intrinsic deposition chamber to substantially prevent unused process gases and nondeposited plasma from contacting the walls of the chamber.